1. Field of the Invention
The present invention relates to the technical field of touch panels and, more particularly, to a low power switching mode driving and sensing method for capacitive multi-touch system.
2. Description of Related Art
The principle of touch panels are based on different sensing manners to detect a voltage, current, acoustic wave, or infrared to thereby detect the coordinate of a touch point on a screen as touched by a finger or other medium. For example, a resistive touch panel uses a potential difference between the upper and lower electrodes to compute the position of a pressed point for detecting the location of the touch point, and a capacitive touch panel uses a capacitance change generated in an electrostatic combination of the arranged transparent electrodes with the touching part of a human body to generate a current or voltage for detecting the coordinate of the touching part.
Upon the principle, the capacitive touch technologies can be divided into a surface capacitive and a projected capacitive sensing. The surface capacitive sensing has a simple configuration, so that the multi-touch implementation is not easy, and the problems of electromagnetic disturbance (EMI) and noises are difficult to be overcome. Therefore, the popular trend of capacitive touch development is toward the projected capacitive sensing.
The projected capacitive sensing can be divided into a self capacitance and a mutual capacitance sensing. The self capacitance sensing indicates that a capacitance coupling is generated between a touch object and a conductor line, and a touch occurrence is decided by measuring a capacitance change of the conductor line. The mutual capacitance sensing indicates that a capacitance coupling is generated between two adjacent conductor lines when a touch occurs.
A typical self capacitance sensing senses the grounded capacitance on every conductor line. Thus, a change of the grounded capacitance is used to determine whether an object is toward the capacitive touch panel. The self capacitance or the grounded capacitance is not a physical capacitor, but parasitic and stray capacitance on every conductor line. FIG. 1 is a schematic view of a typical self capacitance sensing. As shown in FIG. 1, at the first time interval, the driving and sensing devices 110 in a first direction drive the conductor lines in the first direction in order to further charge the self capacitance of the conductor lines in the first direction. At the second period, the driving and sensing devices 110 sense the voltages on the conductor lines in the first direction. At the third period, the driving and sensing devices 120 in a second direction drive the conductor lines in the second direction in order to further charge the self capacitance of the conductor lines in the second direction. At the fourth period, the driving and sensing devices 120 sense the voltages on the conductor lines in the second direction.
The typical self capacitance sensing of FIG. 1 connects both a driver circuit and a sensor circuit on a same conductor line in order to drive the conductor line and sense a signal change on the same conductor line to thereby decide a magnitude of the self capacitance. In this case, the advantages include:
(1) a reduced amount of data since the typical touch panel has m+n data in a single image only, so as to save the hardware cost;
(2) a reduced time required for sensing a touch point since an image raw data can be quickly fetched due to only two sensing operations, i.e., concurrently (or one-by-one) sensing all the conductor lines in the first direction first and then in the second direction, for completing a frame, as well as a relatively reduced time required for converting a sensed signal from analog into digital; and
(3) a lower power consumption due to the reduced amount of data to be processed.
However, such a self capacitance sensing also has the disadvantages as follows:
(1) When there is a floating conductor, such as a water drop, an oil stain, and the like, on the touch panel, it causes an error decision on a touch point.
(2) When there are multiple touch points concurrently on the touch panel, it causes a ghost point effect, so that such a self capacitance sensing cannot be used in multi-touch applications.
Another way of driving the typical capacitive touch panel is to sense a magnitude change of mutual capacitance Cm to thereby determine whether an object is toward the touch panel. Likewise, the mutual capacitance Cm is not a physical capacitor but a mutual capacitance between the conductor lines in the first direction and in the second direction. FIG. 2 is a schematic diagram of a typical mutual capacitance sensing. As shown in FIG. 2, the drivers 210 are located on the first direction (Y), and the sensors 220 are located on the second direction (X). At the upper half of the first time interval T1, the drivers 210 drive the conductor lines 230 in the first direction and use the voltage Vy_1 to charge the mutual capacitance (Cm) 250, and at the lower half, all sensors 220 sense voltages (Vo_1, Vo_2, . . . , Vo_n) on the conductor lines 240 in the second direction to thereby obtain n data. Accordingly, the m×n data can be obtained after m driving periods.
Such a mutual capacitance sensing has the advantages as follows:
(1) It is easily determined whether a touch is generated from a human body since a signal generated from a floating conductor is in a different direction than a grounded conductor.
(2) Every touch point is indicated by a real coordinate, and the real position of each point can be found when multiple points are concurrently touched, so that such a mutual capacitance sensing can easily support the multi-touch applications.
Also, there are some disadvantages as follows:
(1) A single image raw data has an amount of n×m, which is relatively higher than the amount under the self capacitance sensing.
(2) A one-by-one scanning is operated in a selected direction. For example, when there are 20 conductor lines in the first direction (Y), the sensing operation is performed 20 times for obtaining a complete image raw data. Also, due to the large amount of data, the time required for converting a sensed signal from analog into digit is relatively increased.
(3) Due to the large amount of data, the power consumption is thus increased on data processing.
When a touch system is used in a portable device, the optimal use efficiency and allocation is required for the power consumption so as to avoid any waste. The touch system typically has the modes as follows, in order to provide a switch for enhancing the performance to thereby reduce the waste or increasing the use life.
(1) A sleep mode indicates that the touch system enters a state of low power consumption and reduced system resource occupancy when it is inactive or unused for a predetermined long time interval and only the required system wakeup mechanism is remained for the portable device. When the touch system enters in the sleep mode, a specific procedure is required for waking up the touch system to enter in an active/normal mode or idle/inactive mode. In addition, the amount of power consumption in the sleep mode is the lowest among all the modes.
(2) The idle/inactive mode indicates that the touch system enters a state of low power consumption and reduced system resource occupancy when it is inactive or unused for a predetermined short time interval and only the required basic units, which occupy fewer resources and consume less power, are remained in operation, as well as the internal units of the touch system that highly occupy the resources and heavily consume the power are closed. When the touch system enters in the idle/inactive mode, a user can touch the touch system again, so as to allow the touch system to quickly enter in the active/normal mode. Thus, the purpose of saving the unnecessary power consumption is achieved. The amount of power consumption in the idle/inactive mode is lower than that in the active/normal mode.
(3) The active/normal mode indicates that the user can completely use the functions of the touch system so as to quickly response to the use situations of the user. The optimal performance is obtained in the active/normal mode, and the heavy power consumption and the higher system resource occupancy are present. In this case, the amount of power consumption of the touch system in the active/normal mode is the heaviest among all the modes.
Furthermore, whether the self or the mutual capacitance operation is used, a comparison of currently obtained image raw data and base image raw data is required for knowing when a user touches on the touch system. However, the prior art uses a fixed base image raw data to compare with a currently new image raw data, and the fixed base image raw data may loss the accuracy due to the user at different environment conditions or time intervals, so as to lead the touch system to a failure.
Therefore, it is desirable to provide an improved low power switching mode driving and sensing method for capacitive multi-touch system to mitigate and/or obviate the aforementioned problems.